The present invention relates generally to induction motor control, and more particularly to vector control of induction motors.
Control of induction motors can be performed by vector control, otherwise known as field-oriented control. Generally, in vector control of an induction motor, the electric currents in the phases of the motor (i.e., three phases in three-phase motor), are resolved into one xe2x80x9cdirect-axisxe2x80x9d current, id, and one xe2x80x9cquadrature-axisxe2x80x9d current, iq. The direct and quadrature axes reside in a synchronously rotating reference frame.
In vector control, rotor flux of the motor is a function of direct-axis current and is independent of quadrature axis current. Torque produced by the motor is generally a function of both direct-axis and quadrature-axis currents. Rotor flux is a function of only direct-axis current due to selection of the slip speed at which the rotor operates. Slip speed is defined as the difference in rotational speed between the rotor and the electromagnetic field in the stator of the motor. If the slip speed is properly selected, the motor is said to be xe2x80x9cfield-orientedxe2x80x9d and the rotor flux along the quadrature axis is zero. In other words, all of the rotor flux is along the direct axis.
A vector controller chooses desired direct-axis and quadrature-axis currents such that the motor being controlled operates as desired, for example, with the desired torque and speed. Sometimes, in vector control of an induction motor, the controller will assume that the quadrature-axis and direct-axis currents should be equal. For some operating conditions, this assumption will produce good efficiency of the motor being controlled. The controller will then cause the desired quadrature-axis and direct-axis currents to be transformed into three phase currents. The three phase currents are the actual physical electric currents applied to the motor.
The assumption made by the controller that the direct-axis current should be equal to the quadrature-axis current is only a good assumption in certain situations and for certain operating conditions. Sometimes, where saturation of the core of the motor begins to set in, the efficiency resulting from equating quadrature-axis and direct axis currents begins to decrease.
It is known, to help assure high efficiency of an induction motor, to run the motor and measure the efficiency at which the motor is operating. Then, by trial and error, the direct-axis current is modified such that the efficiency of the motor is maintained at a maximum. However, this method is effective only for motors that operate in a few operating conditions. The trial and error approach is not as applicable where motors operate in varying conditions, such as in an electric-vehicle for example.
Typically, an induction motor drive provides three stages of operation. At low speeds, the voltage required by the motor is lower than the voltage capability of the inverter. The output torque is limited by the current capability of the inverter, which is independent of the speed. Accordingly, the first stage of operation below a base speed is often called the constant torque stage of operation. At a medium speed range, above the base speed, the maximum torque can only be achieved when the motor is operated at both voltage and current limits. In this stage, the maximum output torque is inversely proportional to the speed; hence it is called constant power stage. At high speeds, the voltage capability of the inverter is the primary limiting factor for the output torque. The maximum torque is inversely proportional to the square of the speed. It is referred to as the voltage limit stage.
The rotor resistance and the inductance are varied with current and temperature. When the slip frequency is at a nominal value, it produces peak torque at the lowest power. However, the slip frequency is not always at a nominal value. Therefore, it is possible that the flux current can supply too much current, which will overheat the motor. Likewise, it is also possible that the flux current can be too weak, in which case it is not possible to generate enough torque.
It is an object of the present invention to provide a method for efficient operation of an induction motor. It is another object of the present invention to maintain the flux current within predefined limits in order to optimize the motor output.
It is a further object of the present invention to anticipate the rotor flux for optimum motor output.
In carrying out the above objects and other objects and features of the present invention, a method is provided that achieves peak torque in the motor system at all times. The method of the present invention calculates the d-axis current as a function of a constant slip frequency.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.